The invention disclosed and claimed herein generally pertains to a segmented k-space method for three-dimensional (3D) magnetic resonance (MR) imaging. More particularly, the invention pertains to a method of the above type for acquiring MR images of a patient or other subject which is disposed to experience cyclical, or substantially periodic, motion. Even more particularly, the invention pertains to a method of the above type wherein MR data of lower spatial frequencies may be acquired at any specified time during the motion cycle.
As is well known, cyclical or substantially periodic motion in a subject, such as respiratory or cardiac motion, can cause serious artifacts or other distortions in MR images. To reduce such artifacts in acquiring 3D images of the heart, two different techniques for cardiac gated 3D imaging have been developed. In one technique, the phase-encoding direction is played as the outer loop and the slice-encoding direction is played as the inner loop. In each cardiac cycle, all of the slice-encodings of one of the phase-encoding steps are acquired. The entire scan time is then the product of the total number of phase-encoding steps and the cardiac cycle duration. In the second technique, in order to achieve shorter scan times, the phase-encoding is the inner loop and the slice-encoding is the outer loop. During each cardiac cycle, all of the phase-encodings for one of the slice-encodings are acquired. The entire scan time is then the product of the number of slices and the cardiac cycle duration. For this technique, the amount of time to acquire all the phase-encodings for a single slice-encoding must be less than the available imaging time in each cardiac cycle. Thus, it is seen that the above techniques are limited, either by the total number of phase-encodings, or by the total number of slices. It is believed that a more general technique of 3D cardiac imaging is needed, that allows more flexible choice of scan time by allowing any number of phase/slice encodings within the cycle motion.
Segmented k-space is an alternative approach for acquiring MR images of a subject disposed to experience periodic motion. In segmented k-space, the acquisition is divided into multiple segments, each acquired during a different motion cycle. The only restriction on the number of views, or samples, which are acquired per segment is that such number, multiplied by the sequence repetition time, must be no greater than the available imaging time in each cardiac cycle. In conventional two-dimensional (2D) segmented k-space imaging, each segment is assigned a number. Respective views for a given segment are acquired during a number of sequence repetitions, which occur at specified intervals over the motion cycle to which the given segment corresponds. Usefully, a look-up table is provided that maps or orders a particular segment number and repetition thereof to a particular phase-encoding value, associated with a particular view. One example of a phase-encoding order is segmented interleave, wherein respective view numbers in each segment are separated by a number of steps equal to the total number of segments. The view numbers are interleaved from segment to segment. Various segmented k-space techniques are described in the prior art, for example, in an article by Edelman et al, entitled "Segmented TurboFLASH: Method for Breath-Hold MR Imaging of the Liver with Flexible Contrast", Radiology 177:515-521 (1990).
In the construction of an MR image, the views or data samples which are most useful are those acquired close to the center or origin of k-space, that is, which are associated with low spatial frequencies. For example, on the order of 80% of the MR signal contributed to a reconstructed image may be provided by views acquired from the 10% of k-space which is centered at the origin thereof. In segmented k-space techniques of the type described above, MR samples which are of low-order k-space values are acquired only at the beginning of each motion cycle. As a result, such techniques may be seriously limited, if used to acquire cardiac images or the like at other times during the cardiac cycle. As is well known by those of skill in the art, motion of heart structure, such as a coronary artery, has two phases or motion components during a cardiac cycle, i.e., motion during systole (mechanical contraction of the heart) and motion during diastole (mechanical relaxation of dilation of the heart). It may, for example, be desired to acquire an MR image of the heart structure at the approximate mid-point of the systole motion cycle. At such time, the heart tends to be fairly stable. However, if a prior art segmented k-space technique were to be used, it would be necessary to construct such image primarily from higher spatial frequency k-space samples, rather than from the much more useful low frequency samples located close to the k-space center.